Remodeling of the extracellular matrix is necessary for tumor cells to invade and metastasize. Matrix metalloproteinases (MMP) are widely accepted as the proteinases responsible for ECM remodeling. Although MMP are important for wound repair, growth and development, work from many laboratories has implicated increased MMP activity in the process of tumor cell invasion and metastasis. MMP activity is controlled by a family of protein inhibitors called Tissue Inhibitors of Metalloproteinases (TIMP). TIMPs have been shown to decrease the invasive potential of tumor cells in vitro and in vivo suggesting that the invasive potential of tumor cells may be modulated by inhibition of metalloproteinases. Therefore, understanding the mechanisms of metalloproteinase inhibition may lead to new therapies to fight tumor cell invasion and metastasis. In addition to the known TIMPs, a metalloproteinase inhibitor of molecular weight 16,500 has been observed in medium conditioned by some human brain tumors and transformed epithelial cells. Recently, this inhibitor was purified from brain tumor cells and identified as the C-terminus of a protein originally identified as procollagen C-terminal proteinase enhancer (PCPE). This inhibitor activity has been designated CT-PCPE. Inhibitor activity has only been associated with the C-terminus of PCPE. Intact PCPE, a 55 kDa glycoprotein, is not a metalloproteinase inhibitor. Intact PCPE and N-terminal 34 kDa and 36 kDa fragments enhance the activity of procollagen C-terminal proteinase (PCP). CT-PCPE inhibits MMP-2 and MMP-7 in solution with an IC-5O value of 560 nM and 50 nM respectively. However, it is unknown if MMPs are the only metalloproteinase targets for CT-PCPE inhibition. Experiments are proposed to identify the structural components of CT-PCPE responsible for metalloproteinase inhibition and to determine which metalloproteinases constitute physiological targets. In addition, experiments are proposed to identify proteinases responsible for the generation of CT-PCPE inhibitory activity and to determine if both enhancer and inhibitor activities can be released from PCPE by the same proteolytic event. These experiments will provide information regarding the control of metalloproteinase activity in tumor biology.